Multilayer optical films, whose reflection and transmission characteristics are based exclusively or predominantly on constructive and destructive interference of light reflected from interfaces between a plurality or stack of optically thin layers (“microlayers”) within the film, are known. For example, it has long been known to make high reflectivity mirror films by vacuum deposition of alternating layers of inorganic optical materials, such as titanium dioxide (TiO2) and silicon dioxide (SiO2), onto a substrate.
It is also known to provide multilayer optical films with significant in-plane birefringence by coextruding a plurality of alternating polymer layers and stretching the cast web under conditions suitable to thin the cast layers and provide some of the resulting microlayers with stress-induced birefringence. See, e.g., U.S. Pat. Nos. 3,610,729 (Rogers), 4,446,305 (Rogers et al.), and 5,486,949 (Schrenk et al.). The material properties and process conditions are selected so that the stress-induced birefringence provides a refractive index mismatch between adjacent microlayers along one in-plane axis, and a substantial refractive index match along an orthogonal in-plane axis. The index mismatch provides high reflectivity for light polarized along the first axis (block axis), and the index match provides low reflectivity and high transmission for light polarized along the orthogonal axis (pass axis), resulting in a convenient reflective polarizer article.
Recently, researchers from 3M Company have pointed out the significance of layer-to-layer refractive index characteristics of such films along the direction perpendicular to the film, i.e. the z-axis, and shown how these characteristics play an important role in the reflectivity and transmission of the films at oblique angles of incidence. See, e.g., U.S. Pat. No. 5,882,774 (Jonza et al.). Jonza et al. teach, among other things, how a z-axis mismatch in refractive index between adjacent microlayers, more briefly termed the z-index mismatch or Δnz, can be tailored to allow the construction of multilayer stacks for which the Brewster angle (the angle at which reflectance of p-polarized light at an interface goes to zero) is very large or is nonexistent. This in turn allows for the construction of multilayer mirrors and polarizers whose interfacial reflectivity for p-polarized light decreases slowly with increasing angle of incidence, or is independent of angle of incidence, or increases with angle of incidence away from the normal direction. As a result, multilayer films having high reflectivity for both s- and p-polarized light for any incident direction in the case of mirrors, and for the selected direction in the case of polarizers, over a wide bandwidth, can be achieved.